1. Field of the Invention
The present invention relates to an image processing apparatus and control method thereof.
2. Description of the Related Art
An electrographic image forming apparatus has conventionally been known. The electrographic image forming apparatus includes a laser source for scanning (irradiating) a photosensitive drum with a laser beam corresponding to image data, a photosensitive drum irradiated with a laser beam emitted from the laser source, and a charging roller for electrically charging the photosensitive drum. The electrographic image forming apparatus also includes a developer for developing, with toner, an electrostatic latent image on the photosensitive drum, which is formed by irradiating the charged photosensitive drum with a laser beam. The electrographic image forming apparatus further includes a transfer unit for transferring, onto a sheet (print paper), a toner image developed by the developer on the photosensitive drum.
In this image forming apparatus, when the photosensitive drum is scanned (irradiated) with a laser beam based on 1-line image data, the scanning line is ideally a straight line parallel to the axial direction of the photosensitive drum. However, the scanning line sometimes fails to form a straight line parallel to the scanning direction (i.e., the scanning line curves) owing to a mechanical error of the laser source or photosensitive drum attaching position or the like.
FIG. 26 depicts a view illustrating an example of the curvature (registration error) of a scanning line on a photosensitive drum.
In FIG. 26, a dotted line 2600 represents an ideal scanning line of a laser beam. A solid line 2601 represents an actual scanning line of a laser beam.
As a method for correcting the curvature of a scanning line, for example, the following techniques are proposed.
According to Japanese Patent Laid-Open No. 2003-241131, when assembling an exposure unit into an image forming apparatus main body, the inclination of a scanning line is measured using an optical sensor. The exposure unit is mechanically inclined in accordance with the inclination, adjusting the inclination of the scanning line.
According to Japanese Patent Laid-Open No. 2004-170755, the inclination and skew of a scanning line in an image forming apparatus are measured using an optical sensor. Bitmap image data is corrected to cancel the inclination and skew. An image is formed based on the corrected image data. The method in Japanese Patent Laid-Open No. 2004-170755 requires neither mechanical adjustment nor an adjustment process in assembly because the inclination and skew of a scanning line are corrected by image data. Compared to the method disclosed in Japanese Patent Laid-Open No. 2003-241131, the method disclosed in Japanese Patent Laid-Open No. 2004-170755 can cancel the inclination and skew of a scanning line at lower cost.
However, the method disclosed in Japanese Patent Laid-Open No. 2004-170755 requires a line buffer for storing bitmap image data of a plurality of scanning lines because blend processing is executed to blend data of a target scanning line and an upper or lower adjacent line at a skew of the scanning line, and print them. The number of line buffers depends on the skew width of the scanning line. For example, when the skew width of the scanning line corresponds to image data of N lines, line buffers capable of storing image data of N lines are needed. Since the skew width differs between image forming apparatuses, the number N of line buffers must be the number of lines exceeding the maximum value of the skew width. Ensuring many line buffers increases the memory capacity and a circuit scale for correcting bitmap image data, raising the cost.